Well, then I think you're getting into the demographics of the GameCube. RE4 is a Mature-rated game with people's heads popping like grapes. With a system that was so frequently bought for kids, I think it was inevitable that it wouldn't get quite the sales of the family-friendly Nintendo releases, which were bought BOTH by parents for kids and by the older gamers that would pick up RE4.

Also, if you noticed, almost all of the games above RE4 were released between 2001-2003. Fewer games to pick from, so those titles were more likely to be bought by everybody that had the system. By the time RE4 came out, there was a lot to pick from.

So sales aren't really evident of how much people "favor" a game. The gamers I know already had a GameCube before RE4 and bought plenty of first-party titles, but RE4 is still their favorite GameCube game and, had it been out earlier, it would've been a bigger reason for them to pick up the system than the other titles.